Cathode blade for a field emission x-ray tube

ABSTRACT

A cathode blade comprising a thin electron emitting layer formed from tungsten that projects from a copper support layer. The tungsten layer is extremely thin so that electrons will be readily emitted from the edge of the blade without requiring that the portion of the blade proximate the electron emitting edge be tapered. The copper support layer is spaced from the electron emitting edge of the tungsten layer, and therefore, does not reduce the sharpness of that edge. The blade is formed by vapor depositing a thin tungsten layer on a copper layer that is somewhat larger than the desired blade. The copper and tungsten layers are covered with coatings that are unaffected by electroetching processes for removing unwanted portions of those layers. The portions of the copper and tungsten layers having outlines corresponding to the desired outlines of the copper and tungsten layers respectively of the desired blade are masked. The protective coating is removed from the remaining unmasked surfaces. The exposed portion of each layer is then removed by electro-etching.

United States Patent 91 Rogers et al.

[ CATHODE BLADE FOR A FIELD EMISSION X-RAY TUBE [75] Inventors: W. Leslie Rogers, Ann Arbor, Mich.; Ralph C. Ayr es, Mission, Kans.

[73] Assignee: The Bendix Corporation, Southfield,

Mich.

[22] Filed: Dec. 22, 1971 [21] Appl. No.: 210,796

[52] US. Cl. ..3l3/355, 313/56, 313/336 [51] Int. Cl ..H0lj 35/06 [58] Field of Search ..3l3/355, 336, 309,

[56] References Cited UNITED STATES PATENTS 3/1967 Dyke et a1. ..250/l02 5/1972 Maximov... ..313/3ll Attorney-John S. Bell and William S. Thompson [451 May 22,1973

[57] ABSTRACT A cathode blade comprising a thin electron emitting layer formed from tungsten that projects from a copper support layer. The tungsten layer is extremely thin so that electrons will be readily emitted from the edge of the blade without requiring that the portion of the blade proximate the electron emitting edge be tapered. The copper support layer is spaced from the electron emitting edge of the tungsten layer, and therefore, does not reduce the sharpness of that edge. The blade is formed by vapor depositing a thin tungsten layer on a copper layer that is somewhat larger than the desired blade. The copper and tungsten layers are covered with coatings that are unaffected by electro-etching processes for removing unwanted portions of those layers. The portions of the copper and tungsten layers having outlines corresponding to the desired outlines of the copper and tungsten layers respectively of the desired blade are masked. The protective coating is removed from the remaining unmasked surfaces. The exposed portion of each layer is then removed by electro-etching.

4 Claims, 7 Drawing Figures Patented May 22, 1973 wmHl) FIGS CATHODE BLADE FOR A FIELD EMISSION X-RAY TUBE BACKGROUND OF THE INVENTION 1. Field of the Invention X-ray technology.

2. Brief Description of the Prior Art I Field emission x-ray tubes, which are sometimes called cold cathode x-ray tubes, have been known since the l920s. A field emission x-ray tube includes an anode and a cathode disposed in an evacuated envelope. A high electric voltage pulse is supplied to the anode to provide a large potential difference between the anode and cathode. This potential difference causes electrons to flow from the cathode to strike the anode. Electrons striking the anode cause x-rays to be emitted from that anode. The number of x-rays provided, or in other words the output of the x-ray tube,

is determined by the number of electrons that can be made to flow from the cathode to strike the anode. Electrons are more readily emitted from a sharp edge or point than from a flat surface. The electron flow produced by a particular potential difference is increased as the sharpness of the electron emitting edge or point is increased.

A blade shaped cathode in a practical x-ray tube embodiment must have an extremely sharp edge from which electrons are emitted in order to provide an acceptable high x-ray output. The electron emitting edge of cathode blades used in most embodiments must have a radius of curvature between 1-10 and 1'10 inches. Cathode blades having edges with the required degree of sharpness have been produced by tapering the edges of the blades so that they resemble razor blades. However, the required radius of curvature is so small that it is very difficult to accurately control the taper of the cathode blade and provide precisely a determined sharpness. It is also difficult to duplicate a result once obtained and provide a large number of different cathode blades having precisely the same taper so that the electron emitting edges of those blades have precisely the same sharpness. Since the sharpness of the electron emitting edge of the cathode blade determines the tendency of that blade to emit electrons, it is very difficult to produce a large number of tapered blades having the same operating characteristics.

In addition, field emission x-ray tubes operate at such high voltages that a portion of the cathode is vaporized whenever a voltage pulse is supplied to the tube in order to produce x-rays. The vaporization of a tapered blade cathode blunts that blade and therefore changes the operating characteristics of the x-ray tube. The useful life of such a tube is limited because the blade rapidly becomes so blunted that the tube will not provide a sufficient x-ray output.

SUMMARY OF THE INVENTION The subject invention comprises a cathode blade having one lamina or layer that is so thin that there is no requirement that the blade taper toward one edge in order to provide an electron emitting edge that is sufficiently sharp so that the electrons will be readily emitted from that layer. The thinness of this electron emitting layer may vary for different embodiments, but will generally be between H and 1-10" inches. The thickness of the electron emitting layer is the same at each point on that layer so that the sharpness of the electron emitting edge will not change as that blade is vaporized during operation of the x-ray tube. The cathode blade of this invention, therefore, has a relatively long operating life. And, it has relatively uniform operatingcharacteristics throughout that life.

In the preferred embodiment described herein, this layer is formed from tungsten. The electron emitting tungsten layer is so thin that it would not be selfsupporting or structurally rigid over an area equal to the area of a typical cathode blade for a field emission x-ray tube if that thin layer were not supported. The thin electron emitting layer is therefore joined to and supported by a second layer which is formed from cop per in the embodiment illustrated herein. The copper support layer is spaced from the edge of the thin tungsten layer from which electrons are emitted and therefore does not reduce the sharpness of that electron emitting edge.

Since there is no requirement to taper the electron emitting layer of the cathode blade of this invention, that blade can be fabricated using a readily controllable process capable of providing a large number of cathode blades having precisely the same dimension. This process comprises providing a laminae structure having a thin lamina joined to a support lamina. In the embodiment illustrated herein, this laminae structure is provided by vapor depositing a layer of tungsten on a layer of copper. Any portions of the electron emitting and support layers extending beyond the outlines of the electron emitting and support layers of the desired cathode blade are removed in order to produce that blade. This removal of undesired portions includes the removal of any portion of the support layer within a predetermined distance of one edge of the electron emitting layer so that the support layer will not reduce the effective sharpness of this edge. In the embodiment illustrated herein, undesired portions of the layers are eliminated by first coating each side of he laminae structure with a protective coating that will be unaffected by a process such as an electro-etching process for removing unwanted material. Each laminae structure is ten masked with a mask having an outline corresponding to the outline that the lamina masked thereby will have in the finished cathode blade. The protective coating is then removed from all portions of the laminae structure not covered by a mask. The exposed portion of each layer is then removed by an electro-etching process that will not effect the other layer so that no portion of either layer will be removed inadvertently.

The thickness of a layer such as tungsten that is vapor deposited on a support layer of a material such as copper can be very precisely controlled. The abovedescribed process can, therefore, be used to provide a large number of cathode blades in which the electron emitting layers of those blades have precisely the same thicknesses, and in which the edges of those layers have precisely the same radius of curvature. The various cathode blades thus have identical operating characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS Further objects, features and advantages of this invention, which is defined by the appended claims, will become apparent from a consideration of the following description and accompanying drawings in which:

FIG. 1 is a perspective view of one embodiment of the cathode blade of this invention;

FIG. 2 is a schematic, cutaway view of the x-radiating end of an x-ray tube having the cathode blade of FIG. I mounted therein;

FIG. 3 is a perspective view of a two-layer lamina structure from which the cathode blade of FIG.'1 can be produced;

FIG. 4 is an end view of the lamina structure of FIG. 2 with masks having the desired shapes of the various layers of the cathode blade to be formed covering portions of opposite sides of that structure;

FIG. 5 is a plan view of the masked laminar structure of FIG. 4 viewed from the position defined by line 55;

FIG. 6 is a plan view of the masked laminar structure of H6. 4 viewed from the position defined by line 6-6; and

FIG. 7 is a schematic diagram of'apparatus that can be used to remove unwanted portions of the structure shown in FIGS. 2 and 3 by electro-etching and thereby produce the cathode blade shown in FIG. 1.

DETAILED DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates a cathode blade 10 for a field emission x-ray tube. The blade comprises a very thin electron emitting lamina or layer 12 formed from a material such as tungsten. The thinness of layer 12 causes edge 14 of that layer to be extremely sharp so that electrons will be readily emitted from that layer during operation of the x-ray tube. The precise dimensions of layer 12 will vary for different embodiments, but this layer will generally have a thickness between H0 and 110' inches in most embodiments and therefore will have an electron emitting edge with a radius of curvature between l l0 and l-lO inches. A typical cathode blade may have a length along electron emitting edge 14 of approximately 3/16 inch and a length along side 15 of approximately 1/2 inch.

The thinness that layer 12 must have in order to provide an appropriately sharp electron emitting edge 14 is such that the layer would not be self-supporting or structurally rigid over an area as large as the area of a typical cathode blade. Layer 12 is, therefore, joined to and supported by a support layer 16 formed from a material such as copper. Support layer 16 supports only a portion of electron emitting layer 12 and is spaced from the electron emitting edge 14 of layer 12 so that it does not reduce the effective sharpness of that edge. In a typical embodiment, support lamina 16 would be spaced approximately 10 to mils from edge 14. Support layer 16 has a sufficient thickness so that the blade 10 can be mounted in a field emission x-ray tube using conventional mounting techniques such as spot welding or clamping. In a typical embodiment, support layer 16 will have a thickness on the order of I00 times the thickness of electron emitting layer 12.

FIG. 2 is a cutaway illustration of the radiation emitting end of the field emission x-ray tube 18 having four blades 10 mounted in a spaced relationship with respect to the anode 20 of that tube. The blades 10 are held in U-shaped clamps 22 which are spot welded to the walls of the tube 18. The blades 10 are mounted in positions corresponding to the positions of prior art tapered blade cathodes. The blades 10 operate in a mannor similar to that of the prior art tapered blades except that the radius of curvature of edge 14 of the blade 10 will not change as portions of layer 12 are vaporized during operation of the x-ray tube. The blade 10 thus has a long operating life and has relative uniform operating characteristics throughout that life. The operating characteristics of blade 10 will not change substantially until electron emitting layer 12 is vaporized back to the edge of support layer 16.

The blade 10 can be formed from a lamina structure such as structure 24 illustrated in FIG. 3 which comprises a thin lamina or layer 26 of tungsten that has been vapor deposited on a copper support layer 28. Laminae structures such as structure 24 having tungsten layers with a very uniform thickness precisely equal to any desired thickness are commercially available and can be readily purchased. The cathode blade 10 is formed from the lamina structure 24 by first coating each side of that structure with a protective coating that will resist electro-etching processes that affect copper and tungsten. For example, a P type photo resist material may be sprayed and baked onto each side of the lamina structure 24. Masks 30 and 32 (FIGS. 4, 5,

and 6) having outlines identical to the outlines of layers 12 and 16 of blade 10 respectively are placed opposite each other on the surfaces of the tungsten and copper layers of lamina 24 respectively. The masked lamina structure illustrated in FIG. 4 is then placed in an evacuated environment and both sides of that structure are exposed to ultraviolet light. The ultraviolet light breaks down the photo resist material not covered by masks 30 and 32. The masks 30 and 32 are then removed, and the lamina structure 24 is washed in a trichloroethylene solution which removes the portions of the protective photo resist coating that have been exposed to ultraviolet light. The exposed portions of lamina structure 24, that is those portions not covered by the photo resist coating, are then removed by electro-etching.

FIG. 7 illustrates apparatus 34 for electro-etching structure 24. Apparatus 34 includes a container 36 for holding an etching solution, a conductive clamp 37 for holding the structure 24, and a stainless steel cathode plate 38. A voltage source 40 is connected to provide an electric potential between the cathode plate 38 and the lamina structure 24 to be etched. In order to remove the unwanted or exposed portions of the tungsten layer 26, lamina structure 24 and the cathode plate 38 are immersed in a 3 percent by weight sodium hydroxide solution which is placed in container 36. A positive potential of approximately 10 volts is supplied to structure 24 in order to provide a potential difference of approximately 10 volts between that structure and the cathode plate 38. This potential need be supplied for only a very short time interval such as 20 seconds in order to remove all unwanted portions of tungsten layer 26.

The Stoddard electro-etching process is used to remove the unwantcd or exposed portions of copper layer 28. According to this process, the 3 percent sodium hydroxide solution in container 36 is replaced with a solution of chromic acid and sulfuric acid dissolved in water. The solution contains approximately 27 to 35 ounces of chromic acid for each gallon of water, and one part sulfuric acid for each I00 parts chromic acid. The stainless steel cathode plate is replaced by a copper cathode plate. A positive potential of approximately 5 volts is supplied to structure 24 in order to provide a potential difference of approximately 5 volts between that structure and the cathode plate 38 to thereby form the blade 10 by etching away the exposed or unwanted portions of copper layer 28.

Having thus described one embodiment of this invention, a number of modifications will be obvious to those skilled in the art.

Therefore, what is claimed is:

1. In a field emission x-ray tube, an improved cathode blade comprising:

a support lamina; and

an electron emitting lamina being sufficiently thin to lack structural rigidity over an area equal to he area of the cathode blade, said electron emitting lamina being joined to and supported by said support lamina, said electron emitting lamina having an end portion with an electron emitting edge extending from said support lamina.

2. The cathode blade of claim 1 in which the thickness at each point on said electron emitting lamina is substantially equal to the thickness at every other point on said electron emitting lamina.

3. The cathode blade of claim 2 in which said electron emitting lamina is a tungsten lamina having a thickness of approximately M0 to 1-10" inches to eliminate any requirement that the blade taper toward one edge in order to provide an electron emitting edge that is sufficiently sharp so that electrons will be readily emitted from that edge.

4. The cathode blade of claim 3 in which: said electron emitting end portion of said tungsten lamina extends approximately 10 to 20 mils from said support lamina; and said electron emitting edge has a length of approximately three-sixteenths of an inch. 

2. The cathode blade of claim 1 in which the thickness at each point on said electron emitting lamina is substantially equal to the thickness at every other point on said electron emitting lamina.
 3. The cathode blade of claim 2 in which said electron emitting lamina is a tungsten lamina having a thickness of approximately 1.10 4 to 1.10 6 inches to eliminate any requirement that the blade taper toward one edge in order to provide an electron emitting edge that is sufficiently sharp so that electrons will be readily emitted from that edge.
 4. The cathode blade of claim 3 in which: said electron emitting end portion of said tungsten lamina extends approximately 10 to 20 mils from said support lamina; and said electron emitting edge has a length of approximately three-sixteenths of an inch. 